Galaxy s8 plus still dident recieve software update !!

Yeah, maybe, I think it's not such a bad thing, if you have a device that gets patches direct, then it's handy, I was on the 4/5 security patch on my 6P (on 4/6), for example.

I believe they've released the May update now too (at least for non-OEM devices), mine got pushed this morning.

Sure, for OEMs it's tricky, they have to weigh the risks associated with the additional updates, and make a decision accordingly.  This way they have the option, say if there's a particularly nasty security issue that was found, they can do a one-off push (and hope that carriers follow suit).

Maybe, someday, they'll move away from this nonsense for at least minor updates/patches, and go direct, if they're targeted at core components (which are pretty much not changed by OEMs).