It might be forgotten now, but Office was a trade show for new ideas in Microsoft. Its UI routinely used non-standard controls, and relied on knowing how things

It might be forgotten now, but Office was a trade show for new ideas in Microsoft. Its UI routinely used non-standard controls, and relied on knowing how things move under the hood. On the other hand, it was one of the most important applications which had to remain compatible with any Windows system. There is probably an enormous amount of Office-specific fixes in Windows, and that complex and brittle symbiosis requires continuous work to function. There is a multi-dimensional community of people who deal with old software and old hardware. It has noticed that compatibility of Windows 10 with old versions of Office got worse in releases made in 2018. It seems that those old Office versions were finally removed from integration testing environments at that time. Problems are different in different applications (Excel might be the most sensitive). Problems may depend on language version. Problems may only affect certain actions (say, crashing during spelling checks). Problems may depend on updates installed — and certain official updates were known to break some applications for some people even before that, so they certainly were not as thoroughly tested as regular ones. It is great if everything works for you. It is also possible that your specific combination of COM objects and silently overloaded SxS libraries from multiple Office versions works, while some other combination wouldn't.