from the man page you linked to. And none of /run/systemd/generator{,.late,.early} seem like places to place .network or .netdev units?
To the extent that Netplan's sd-networkd generator does not produce systemd units, it shouldn't be [treated as] a generator in the first place. It only needs to run before sd-networkd and write its stuff to /run/systemd/network.
If it also produces systemd units then *that* part of its job is
covered by the systemd.generator manpage; if not, then it doesn't
even need to be a generator — it only needs to run before
sd-networkd, which is easy enough to set up (add a drop-in unit
with ExecStartPre= stanza).
-- -- regards -- -- Matthias Urlichs
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