After suggested modification to SLIME and NAMED-READATBLES, explicit in-readtable form is required at every file
in my understanding that has always been the published API/contract of IN-READTABLE.
and also every time we issue in-package form in REPL.
again, in my understanding IN-READTABLE and IN-PACKAGE were meant to be independent. it's only a kludge that the implementation was intertwined for a while, because proper implementation required slime supporting IN-READTABLE similar to how it supports IN-PACKAGE. no one got to implement it before you, so the *READTABLE-ALIST* based kludge made things work good enough for the time being.
i think catering for user code that relied on the sideffect of that kludge is not a priority.