on osu!'s
terms of service (in which you're prompted to agree upon registering your account) it has been stated that "osu! assumes no ownership of user-uploaded contents unless otherwise specified" and that "you are responsible for stuffs that you put into the site" so yeah, uploading beatmaps of copyrighted songs without a proper clearance from the respective rights holders beforehand are not advised in any situation 😦
if you choose to go for it anyway, then yeah as others have said you're subjected to a potential dmca takedown at any time (
you can read some examples of dmca takedowns that osu! has received here). osu! will then be obligated to take the infringing materials down once osu! received such claim (see :
Cubbby's Arche for example).
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that being said however, there are many artists out there (even those who are not listed in the Featured Artist page) who are very cooperative and open about their song(s) being mapped in osu! tho (see :
(1) (2) (3) for some examples).
as hard as it sounds, getting permission from them might not be as long of a shot as you first think
