Would it be possible to run the deck validation logic against multiple formats?

I have a deck that is modern legal; however, it gets validated as extended legal, which is accurate as well.

The "smallest" format is displayed, because other wise it would make little sens.
For example all standard decks are regal in every other format as well (not counting the banning), so there is no point telling the user that their standard deck is also legal in modern, extended, legacy etc.

So this request is not very useful, you should learn the format types and how they contain each other (like in math N < Z < Q < R :P )

Correct, at the time of my writing the format legality code, there was a strict rule of Standard decks being legal in everything, Extended being legal also in everything except Standard etc. So Standard < Extended < Modern < Vintage < Legacy.

I know there have been (maybe still are?) some exceptions to this, but I've never looked very closely at the implications. For the sake of simplicity I decided to just show one.

Spoook wrote: So this request is not very useful, you should learn the format types and how they contain each other (like in math N < Z < Q < R :P )

Actually it is very useful since your statement is no longer correct and has not always been correct.

There are many cards in standard that are not legal in older formats now.

With the trend continuing with how bans are made it would be useful to make sure the format we intend to play is auto validated.

I don't play extended; however, my deck is extended legal. I'm more concerned if it is modern legal.