My group recently migrated from classic to New Google Sites. There were many things I was doing with classic sites and/or apps script's Sites Service I can't do
positive-lookahead
featuretoggle
upnpx
zoneminder
git-filter-repo
re-frame
django-1.9
fts5
big-ip
bgtaskscheduler
wevtutil
hacker-news-api
mpmediaitem
partiql
content-editor
selecteditem
session-storage
powerpivot
ggfortify
compilation
crtdbg.h
azure-compute-emulator
scala-repl
hashmap
libtool
css-content
gunzip
identity-aware-proxy
xna
google-cardboard