I would say it's the same question as "what does IRC do?" as the general design is the same - maybe a bit more friction signing up with Matrix than IRC but at the end of the road they operate on the same trust model. The Matrix team blogged about a spambot attack last year on this very question: https://matrix.org/blog/2021/06/30/security-update-synapse-1...

Side comment: because my mobile number was leaked (breached) by T-Mobile which included my name, I get way, way way more spam via it than anything else. Lots of political spam, 95% or greater from the right/repub end of the spectrum.

Edit, side comment #2: I ported my secondary Google Voice number out to Tello (a low cost MVNO in the US) with a SIM in a spare phone, and all the spam I was getting every day to that number simply stopped instantly. I'm a little amazed to be honest, it's very interesting.