I was recently testing using a 5G device (it does 5G, 2.5G and 1G only
unlike the 6 speeds of Mikrotik's S+RJ10) When connected to a remote
device at 5G and 2.5G it requires the backend to be running at 10G. And
advertises (according to RouterOS): 1G baseX and 10G base SR LR. However
when the remote device was set for 1G it stops advertising the 10G rate,
and only advertises 1G baseX, and will only connect if the backend is
running at 1G.

In Auto Negotiate mode, the router nicely went from 10G to 1G and back.

I was a bit surprised by this, I had previously assumed the SFP(+)
device advertised rates were fixed. I assume this device might have had
to notify the router somehow of the change in speed advertisements as
well.

However it does explain something I have wondered about previously. The
documentation for the S+RJ10 has always stated the router must run in
Auto negotiation mode for these devices, and I never understood how that
was going to make any difference. Now I do.