It's certainly something I have considered before, but has quite a few implications.

Persistable tracker settings that drastically change how a song is meant to be played are a big no-no.
We've already had that in the past (like reverb settings in (Open)MPT, or even stuff like Fasttracker 2's sound card amplification setting) and it just leads to modules that are unplayable in practice, unless you preceisely read their module texts.

Adding yet another tempo mode to legacy formats also feels wrong. We'd be going back to adding yet another feature that is not playable by most other non-libopenmpt players. It could be something that's only active during an editing session and then automatically is saved as "normal" classic tempo mode in IT/XM files, but how useful would that be?

Is accurate tempo a drastic enough change to not be a persistent tracker setting?

As for compatibility with other players, not all players implement FT2/IT's inaccurate tempo anyway, so it doesn't seem like a huge issue to me.

Is accurate tempo a drastic enough change to not be a persistent tracker setting?

As a tracker setting, it's just a terrible choice if now almost every module out there which uses drum loops etc. will sound slightly wrong because a person accidentally enabled the setting without understanding what it means, and in return every module they release will have the wrong tempo for everyone else.

As for compatibility with other players, not all players implement FT2/IT's inaccurate tempo anyway, so it doesn't seem like a huge issue to me.

Which players use precise tempo calculations? I'm not aware of any. The way FT2 or IT implement tempo was the obvious choice when the majority of player engines were built.

drum loops

Fair. :)

Which players use precise tempo calculations?

I think FMOD, but I might be misremembering.

It could be something that's only active during an editing session and then automatically is saved as "normal" classic tempo mode in IT/XM files, but how useful would that be?

That would certainly be useful for rendering and importing into a modern DAW, but what about doing the opposite (importing a song with accurate tempo as a sample)? In that case, saving and reloading the module would mess things up.