I don't believe there has been a decrease in full-nodes since 2018. According to these stats - https://bitnodes.io/dashboard/7y/ - the total number of listening full nodes, including Tor, has risen from 12,000 on Jan 2018 to 13,500 now. There has been a decrease in the total number of IPv4/IPv6 nodes. But that probably represents people switching to running their nodes over Tor as more and more off-the-shelf node packages support that. Indeed, many are Tor-only.

Furthermore, remember that those stats do not include all full-nodes: many full-nodes are unreachable as they don't listen on any publicly accessible method like IPv4/IPv6/Tor/etc. Those nodes are still important, as they provide their owners the ability to fully verify their transactions. The Bitcoin P2P network has plenty of spare bandwidth, and if needed, I'm sure we could get more listening nodes quickly.

Luke-Jr's data is very imprecise, as it relies on gossipped node address advertisements. They can be trivially faked, and we have no idea if that is or is not happening.