What opcodes are supported in Liquid as of today (September 2021) but not (yet) on Bitcoin? What is the rationale for including these particular opcodes but not others?
What opcodes are supported in Liquid but not (yet perhaps) on Bitcoin?
Burak answered this on Telegram. Here is a list of those opcodes:
OP_CHECKSIGFROMSTACK OP_CAT OP_SUBSTR OP_DETERMINISTICRANDOM OP_LSHIFT OP_RSHIFT
With OP_CAT and OP_CHECKSIGFROMSTACK you can make covenants on Liquid as described in this blog post from Andrew Poelstra. Using these particular opcodes for covenants seem to be considered less introspective and expensive than some alternative proposed opcodes for covenants (e.g. OP_CTV).
There are a bunch of additional opcodes that are proposed to be added to Liquid as part of the Tapscript upgrade but won't be supported on Bitcoin post Taproot activation. For more details on those see here.