With the code
\documentclass{article} \usepackage{etoolbox} \newenvironment{Toto}{% }{% } \AfterEndEnvironment{Toto}{I forgot\dots} \begin{document} \begin{Toto} \end{Toto} \csundef{@afterend@Toto@hook} \begin{Toto} \end{Toto} \end{document} \csundef doesn't work with TL2020 + 2020/10/05 v2.5k e-TeX tools for LaTeX (JAW)
but in overleaf, it works with TL2020 + etoolbox 2020/07/15 v2.5i e-TeX tools for LaTeX (JAW)
I don't think this has ever been the right way to remove something from \AfterEndEnvironment.
\documentclass{article} \usepackage{etoolbox} \newenvironment{Toto}{% }{% } \newcommand{\iforgot}{I forgot\dots} \AfterEndEnvironment{Toto}{\iforgot} \begin{document} \begin{Toto} x \end{Toto} \renewcommand{\iforgot}{} \begin{Toto} x \end{Toto} \end{document} A new hook management is now built in the LaTeX kernel from the 2020-10-01 release.
\documentclass{article} \newenvironment{Toto}{}{} \AddToHook{env/Toto/after}[forgot]{I forgot\ldots} \begin{document} \begin{Toto} x \end{Toto} \RemoveFromHook{env/Toto/after}[forgot] \begin{Toto} x \end{Toto} \end{document} The label in square brackets is an arbitrary string. You can add as many pieces of code you want and only disable some of them through the label mechanism. See the documentation with
texdoc lthooks 
etoolboxhas been updated to be aware of this