it have been exist since 2015 and it still updating to this day and haven't been shutdown and AMXX has been compatable with it since 2017(?) so it is safe to say it is ok to use
BUT
If you search a bit, you will come across this post saying reverse engineering GoldSrc is illegal officially by Vavle staff
https://forums.alliedmods.net/showthread.php?p=2546851
BUTx2
Many of the module in AMXX were created by reverse-engineering HLDS so who know?