While updating/upgrading my librem15, I keep seeing the following message,
The following packages have been kept back:
pureos-standard
Is this a meta package? When I try an apt full-upgrade, it shows the following,
The following packages will be REMOVED:
ntp
The following NEW packages will be installed:
systemd-timesyncd
The following packages will be upgraded:
pureos-standard
does it mean that pureos is using a new package for time sync on networks?
Basically yes. (Although I thought they have been using systemd-timesyncd for a long time, some years. But maybe that was previously only on the Librem 5. Or maybe you have been ignoring the message for a long time. 
)
If you are not using ntp, and you are not wedded to ntp as distinct from systemd-timesyncd, then you can go ahead and do the full-upgrade. However in that case it will remove ntp.
If you do apt upgrade then it will never gratuitously remove packages and hence if the upgrade of a specific package requires removing some other package then the upgrade of the first package will be held back.
I think for most people they won’t care how the system time gets kept up to date, as long as it happens!
yes, that’s quite possible