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I recently learnt a rule in STP: only Designated ports forward BPDUs.

However, I'm not sure about this in RSTP since ALL switches generate their own BPDUs every few seconds. So, in a certain LAN topology a switch may have a root port and an alternate port and no designated port.

Does this rule still apply? Or will switches in RSTP forward their own BPDUs via their root port or alternate port as well?

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BPDUs are not forwarded, generally.

BPDUs are generated and sent on all links, including blocked ones.

Topology information contained in the BPDUs can be considered to be forwarded, but the BPDUs themselves aren't.

"Dumb" (unmanaged) switches often forward BPDUs even though they're not suppposed to - see IEEE 802.1Q 8.6.3:

The [...] group address is an address that no conformant [...] or MAC Bridge can forward.

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