Dropbear difficulties due to outdated version?

[James Miller]

I set up a small low-resource VPS a few years ago to use mainly as a 
light-use xmpp server. I got Dropbear operating there so I could admin it. 
Dropbear seemed a good choice since system resources were so anemic. I 
recall it being quite challenging to get key-pair authentication to 
finally work there, though I can't recall many details about how I finally 
succeeded.

Sometime during the interval after I set that up, key-pair authentication 
has stopped working again. So I've had to re-enable username/password 
authentication. I'm now trying to determine why it happened that the 
key-pair authentication stopped working and am hoping I can somehow 
re-enable it.

The VPS runs Ubuntu 16.04 (EMS), so the version of Dropbear there is a bit 
outdated (v2017.75). Since that release was made, various changes have 
happened to openssh that may, I assume, make it incompatible with this 
version of Dropbear. I am using ssh when I try to connect to the VPS, btw.

So I'd like to start off by just asking whether the challenges I am 
encountering as I try to re-enable key-pair authentication are likely 
related to the version mismatch between the Dropbear I have installed on 
the VPS and the much more current version of openssh I have on the home 
computer from which I'm trying to connect?

Input on that question will be appreciated. If it seems unreasonable to 
expect modern versions of openssh to interoperate with a Dropbear server 
dating to 2017, I will need to just give up on it and seek some other 
solution.

Thank you