modern operating system running on a Reagan era computer

Something OP put in the title as clickbait and it's not even accurate. Ronald Reagan went out of office in 1989 while the video clearly states the OS used needs at least an Intel Pentium processor which was released in 1993.

The term of a US president in the early 1980s.

Yeah it's not Linux. It's forked off MenuetOS (https://menuetos.net/ ) which is a hobby OS written entirely in assembly (FASM flavor, https://flatassembler.net/ ).

All the bloat is a function of people not caring, and to some degree different requirements.

At the operating system level I would say the different requirements are probably the more relevant reason than people not caring. Modern operating systems, even ones optimized for weaker hardware, include a lot of stuff that was either not invented or at least not common in consumer PCs in the 80s and early 90s:

• wifi
• several encryption standards, especially TLS
• many other network-related features; we're talking about a time before HTTP and most other modern protocols were invented
• support for multi-core CPUs or even multi-tasking at all
• modern memory management (paging, virtual memory, swapping...)
• USB
• proper audio output instead of just a few predefined channels
• much more complicated video output
• drivers for all the most common hardware so you can just install it and go
• ...

And that's not even talking about people expecting a modern-looking desktop environment, web browser, file browser, image viewer, text editor and so on. The linux kernel is modular so if you compile your own, you can disable a lot of stuff that you don't need to save space and with something written specifically for those old machines, you can strip out even more stuff you don't need.

That being said, the thread title is very misleading. In the video he states the OS he uses works on the original Pentium processor which came out in 1993. Four years after Reagan went out of office.