As a gamer, I’m going to assume RTS stands for Real-Time Strategy.
I read somewhere a long time ago that just because a term is in the form of an acronym, that doesn’t change what comes before it. So in this case, it’s proper to use ‘a’ rather than ‘an.’
However, I acknowledge that saying ‘a RTS’ sounds icky, so I tend to use the “incorrect,” ‘an RTS.’
I'd rather see the vowel/consonant arbitrary rule rather than the subjective 'what does pronunciation sound like.' Ultimately, written language rules are determined by the consensus of use, not the other way around. In 1k years, the pronunciation will be obscured while the rule will be deterministic and obvious.