readonly=true is used to make an external special remote that does not
need the external program to be installed. It was stored in the
remote.log by default, and so every time it was specified in an
enableremote or initremote, whatever value was used became the new
default for subsequent enableremotes of that remote.
That was surprising, and I consider it to be a bug.
It does not make much sense to pass it to initremote because then how
would you populate that remote with anything? You would have to
enableremote elsewhere, and store content there. I'm assuming nobody
used it that way.
Someone might rely on passing it to enableremote once, and then that
being inherited in other clones. But that is not how it's documented to
be used. It is barely documented in git-annex at all, only in the
external special remote protocol, and the documentation there says to
"Document that this external special remote can be used in readonly
mode." (by the user of it passing readonly=true to enableremote). The
one external special remote that I know of that does document that is
<https://github.com/bgilbert/gcsannex> (the one that motivated adding
it). That one's docs do say to pass it to enableremote.
So, it seemed safe to make this behavior change. If someone was in fact
relying on one of those behaviors, all their current repos will still
work as they configured them (although they will need to deal
with the related change in 9f3c2dfeda).
In new clones, they will find enableremote fails, complaining the
external program is not in path. An easy enough problem to recover from.
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