Alas, trying to support buffer and polybuffer and make it essentially transparent in Max was an overreach on my part: it was just too gnarly to have that and have stable behaviour.
The tricky thing is that, when all you have is a symbol and the patch hasn’t finished building, it’s pretty hard to work out whether you have a buffer or a polybuffer to deal with.
If people feel that they really miss this, I see two possibilities we could explore in the future. First, bring them back, but without trying to make in transparent in Max (that way the code knows what it’s meant to be doing). Or, when I implement array attributes (mostly to try and tame BufCompose!), then some mapping from polybuffer to array of buffer might be possible.