Are modern receivers any better than older ones?

Yes modern everything is better, though it is hard to compare apples and apples in this case.  A lot has changed!

The OP main concern is sound quality. He wants to know if moder AVR will sound better than ancient, perhaps decade old AVR.

  This is a complex question.  If the AVR is going to be used purely as an amplifier, probably not.  Most AVR use Class D amplification now.  The implementation of Class D has improved dramatically from 15 years or so ago, but if your old AVR uses Class AB, it will probably sound better in the amplification section.

  However, AVRs are also frequently DACs, depending upon your digital source.  The DACs in AVRs have come a long way in the past decade.

  Finally, there is Room Correction, which some AVRs do quite well, such as Anthem and Yamaha.  The OP may view RC as a “feature “ but since it can fundamentally improve the sound, it is an important one

I wanna know if modern receivers offer any better sound quality? or do they just have more inputs/music streaming/other random features.

I currently have an old Sony STR-DE898, and I’m debating whether it’s worth upgrading or not, I don’t really care about extra features and stuff, I just want the best sound quality https://100001.onl/ https://1921681254.mx/.

And if I’m upgrading, what specs should I be looking at for a new receiver?

I got this,...

Depends on the specific models you are comparing I think. I would only get a new receiver if i was into home theater and needed dolby atmos, otherwise your money is better spent on music geared seperates for pure SQ.