My collection:
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I'll be very disappointed if we miss traditional retrospective GTK2 themes - see this thread for details - Problems with GTK2 themes (such as human-theme) on Ubuntu 18.04 LTS MATE + poll .
There is a working solution on AskUbuntu about Human and Human Clearlooks themes, but not for other themes.
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Problems with scrolling of applications menu and drop down menu in workspace switcher. FIxed in GTK3 sources, but currently not in eoan repository.
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Caja does not show previews of text files on Bionic and newer versions.