FWIW, I think uncertain length lookbehind support was recently added in 2023 to PCRE2.
https://github.com/PCRE2Project/pcre2/issues/269
But if that is the only feature you are interested in (ie. all plugins, action lists, epub3 support, multi-language spellcheck, saved search controls, replace all selection from tables, dark and light theme support, and even the upcoming python regex function replace, etc... are really not important to you) then you should probably stay with Sigil 0.7.2, as things have changed significantly. ie. there is no BookView (it is now in a separate app called PageEdit) and the search routines have been changed heavily to keep track of more state information so that a new restart search button is needed, and the ui has expanded considerably.
And I agree with DiapDealer backing up your current Sigil preferences folder would be essential for ever going back to Sigil 0.7.2