Like dude above said, some do, some dont. it really all depends on the artist/producer relationship or the producers name. for example, lex has reached the point where he can charge everybody for a mixtape track. he doesnt need the artist, the artist wnts his sound and the name attached to it. i got a track on The Marathon Continues mixtape by Nipsey Hussle (10 toes) and i didnt get paid for it. to be honest neither he nor anyone in his squad contacted me to let me know they were using the beat. i found out a few days before the tape dropped and thats only because i heard my beat playing in the background of an interview he did. if u listen u can hear my tag all through out the beat.
now if he had contacted me i would have gladly set him the untagged version of the track but idk what happened with that. i can still sell that beat if i wanna. he released an instrumental and acapella version of the mixtape but 10 toes wasnt on there.
but im to the point where im not chasing mixtape placements. if run across an artist i believe in or support ill throw them a track for free but free doesnt pay the bills. im chasing after paid placements, got a few in the works right now. because unless your a household name theres no guarantee an artist will want to pay u for ur track, and their argument will be "well if i use this track on my mixtape it will help get ur name out to more people" and 9 times out of 10 u will oblige with that and run with it because u want exposure.
the only way to guarantee u will get paid for ur track is if its gonna get pushed as a single or gonna get placed on an album