Since the last Proxmox Update V9.1.14 the following message appears for migration tasks:
Plugin “PVE::Storage::Custom::LINSTORPlugin” is implementing an older storage API, an upgrade is recommended
Where to find the upgrade?
this is usually just harmless and as soon as I get aware I release a new version officially supporting the new version with in a day or two. thanks for noticing, I will check what changed and if things are harmless as usual…
yes, completely harmless, just ignore this warning until a new plugin version is out
Thanks for the quick response and fresh update 
Proxmox deployed a new kernel last week - same warning again.
Maybe you have an eye on.
Thanks
this has absolutely nothing to do with kernel updates. They updated their storage (plugin) API. As usually completely harmless, but I also released 8.2.2 bumping the supported API version. The upcoming 8.3.0 will also support APIVER 15.