Are there any seasons when it is considered "unacceptable" to wear a
tan suit in the office?

If we're talking like a medium tan, wear it anytime.
If we're talking like a darker tan, don't wear that in spring.

Forget the season, though.  The real question is your complexion.  The
world wears simple dark colors (navy/charcoal) because it goes with
most skins and body shapes.  How would you describe your skin tone and
body shape/height?

I'm medium build, average height, w/"average" skin tone. I have dark
brown hair and beard.

It's a medium tan suit. I saw a local store window showing tan suits
paired w/fall color ties (rust tones) and that's what prompted my
question. Thanks.

No. Simple as that