@cartpaujSilent Post has been deprecated for a good bit now, but it is not end of life and no EOL has been announced. If you still want to use silent post you can, you just have to a) disable hash verification in your code or b) change your md5 verification to the sha512 verification.
The only thing that may affect you is some users have reported no sha512 in the response for ARB. Webhooks will be the way to go in the end. There is no declined payment webhook, but you can keep track of the subscription dates on your end and update accordingly without too much work. I have not had to do this but I do think it will be straightforward.
So maybe your first step is to log some test transaction responses and see if the sha512 hash is in there. If that works, might be best to patch up your silent post url while you work out what to do with webhooks.