Let's run Win10 on really really old hardware

Most inexperienced users upgrade an old laptop to Windows 10 without searching for drivers first. An Intel 915 or earlier system has no WDDM graphics drivers and is slow as hell. Even if there is WDDM graphics driver, Windows Update might not find it automatically and you must to download it manually from Intel or whatever. Even with all drivers properly installed modern Windows 10 versions have more features, meaning more processes running in the background and reducing resources and decreasing performance. So the same system running Windows 10 v21H2 is slower than running Windows 10 v1809, say, or Windows 7. You have to disable some unnecessary services and visual effects to increase performance. See my previous posts.

If I had to choose between running a no longer supported OS, such as Windows 7 and a modern OS such as Windows 10, I would choose the latter, even if it was a little slower. It is not only the absence of security updates, who cares, I would use a third party antivirus for that, it is that some applications either are not available anymore for the old OS, or you need to install a lot of patches to make them work. If the computer can run Windows 8 64-bit, I can also bypass compatibility check and install Windows 11, why not. When a customer's computer needs a new Windows installation, I avoid installing Windows 10 64-bit. I install Windows 11 instead to be as modern as it gets. They will thank me later. Don't worry about drivers. Almost all Windows Vista 64-bit or higher driver works fine in Windows 10 64-bit and in Windows 11.

I'm currently running Windows 10 1607 on an old Dell Inspiron 9300 (Pentium M 1.73 with downvolt using RMClock to keep it cool, ATI x300 WDDM 1.0 graphics chip, 2 GB DDR2 RAM, IDE 160 GB HDD). I used Snappy driver installer to get all the necessary drivers (IIRC it needed only graphics and sound drivers, maybe Wi-Fi too). I disabled CompatTelRunner (and also Defender) from running in the background to keep it from bothering HDD and warming up CPU. I also disabled most of the visual effects and transparency. I use it primarily for some background tasks, so it's pretty fine for this role, and I access it via RDP (which is kind of slow, but tolerable).
I will be upgrading it to m.2 SSD soon using IDE 44-pin to m.2 SATA adapter, and I'd like to try to run a newer 1809 OS build. The last time I tried it, it blue-screened right from WinPE IIRC, but I didn't try very hard to get it working.
Also the fun thing is there is a build of Ungoogled Chromium which runs on SSE2-only CPUs (now outdated, but anyway), which... well, it's slower than Firefox on this machine IMO (not that I do any browsing there).
I considered using a console-only x86 Linux (Debian or Arch32) in the future for my needs, but I still haven't gone this way yet.
Also, there is some file IO CPU bottlenecking in some scenarios introduced in a cumulative update I think, and that's a part of the reason I want to upgrade the OS (I'm not sure if I can post the link to the forum thread about it, and anyway it gone nowhere after a few posts).

I had successfully upgraded a similar (Core Duo) laptop from v1607 to v21H2. Pity the motherboard was damaged and could not install all drivers. So it is possible, create a full backup and try. If anything goes wrong just restore the backup and that's it.

So I've managed to run Windows 10 1809 on my Inspiron 9300.

I tried a bunch of different stuff simultaneously to save time, so I don't know if everything I tried was really necessary.
So here's my journey:
I backed up all third-party drivers from my previous 1607 installation using Double driver (not a perfect solution as I figured later).
Then, I prepared a custom 1809 wim image (esd actually, but try this with caution because LZMS compression consumes gigantic amounts of RAM while compressing an image) with the latest servicing stack and cumulative updates on my main PC, and also integrated all of the drivers.
Then I physically removed a DVD drive from the notebook because a saw a recommendation somewhere to empty every IDE channel except the HDD one in case of BSODs. I also had internal Ethernet NIC and modem disabled because for ethernet I use a slightly faster USB adapter, and modems are useless nowadays.
Then I prepared a USB flash drive with 1607 WinPE which I knew works. I deleted a 1607 install image and copied my modified 1809 wim there. I had to use wimlib-apply instead of a regular Windows install app because the older DISM refuses to work with newer images. I also had to partition my disk manually using diskpart because it was empty at that time.
I rebooted the notebook after applying the image, and it didn't BSOD for some reason! Installation went smoothly until OOBE happened. It was somewhat broken, and it was just restarting itself after selecting a country. Also all USB port were non-functional at that time because the integrated USB controller drivers from 1607 didn't install properly on 1809. So I rebooted into audit mode to bypass OOBE (it's not really necessary to finish OOBE to complete installation) and I also fixed USB by installing stock Microsoft USB drivers. I also disabled the second IDE channel because I thought that was the problem that leads to BSOD.
Then I rebooted the notebook into the desktop. I had to (re)install graphics and sound drivers because they didn't apply successfully for some reason, and also installed all of the offered updates.
I use RMClock which requires UAC elevation, so I created a task to run it automatically as startup with admin privileges. And it didn't work! It turned out that OOBE bypass partially breaks the task scheduler. So I had to relaunch OOBE using sysprep.exe and this time it completed without any bugs and after that the task started working as intended.
Then I tried to figure out what was causing BSODs during the installation. So I enabled internal Ethernet and tried to boot, and the system started fine. Then I re-enabled the second IDE channel and installed the DVD drive back. And again, no BSOD. So at this point I don't know the root cause of it.
The system is fully functional at this point. The one slight bug I've encountered is when the screen turns back on after inactivity timeout, it's blank with only a mouse cursor on it. But tapping Win+P several times to switch displays (imaginary in this case) brings everything back.

Certainly. If it can run Windows 8/10 64-bit it can also run Windows 11, albeit you have to bypass compatibility check. Officially Microsoft checks your computer and asks for Intel 8th generation or later, or AMD Ryzen 2 or later, TPM 2.0, 4GB RAM, Secure Boot and UEFI boot to install Windows 11. But you can bypass compatibility check and install in any Core 2 Duo or later with at least 2GB and in Legacy BIOS (MBR) boot. See respective thread in Windows 11 Forum