There’s feature request that asks about opposite thing. While it’s not marked as implemented it seems that Obsidian works exactly that way.
Obsidian stores spell check exceptions in var/app/md.obsidian.Obsidian/config/obsidian/Custom Dictionary.txt (for Flatpak), but honestly adding every acronym I use into it is too much tedious giving that it’s per-user dictionary, so it’s really hard to sync it across several devices.
The only “workaround” I’ve found is to wrap acronyms to code blocks which (again) too much work. I think ignoring uppercase words is a very reasonable default behavior.
I may be wrong, but at least on Obsidian Mobile, I am noticing that capitalized misspelled words that are 6 characters or fewer are ignored by the spellcheck. I will be interested to check whether this is also true for the desktop version.
When I made the request that you linked to, I am quite certain that capitalized misspelled words were totally ignored by the spellcheck regardless of length. Perhaps the behavior has changed. Although obviously not super urgent, I am still interested in finding a way to toggle capitalized spellcheck.