Is it still worth buying Diablo 2?

No, you should have played it when it was relevant

but you probably weren't even born yet

Some of these older ARPGs haven't aged very well gameplay wise. I recently started playing Titan Quest again, and the gameplay is severely dated. Very marginal low to no cost LMB attacks combined with a load of other stuff on C/Ds arguably too long and/or too high energy cost to be viable for regular usage. More modern ARPGs have solved that problem a number of ways. I will say that D2 aged better than TQ has though.

Games like D3, PoE, Grim Dawn all have superior gameplay to the older ARPGs. Its just a fact, I don't see why anyone would deny it.

OP.
No it's not worth it. The online version of it is pretty dead and is infested with maphackers and bots. I know cos i used them once myself for months and nothing ever happened. The offline version - why would you want to play that...dunno. Besides d2 is not just d2, there's an expansion called "Lord of destruction" too.