This question should be obvious, but I cannot find the answer online and have no way of checking for myself with my very minimal LaTeX knowledge. Does Overleaf (by default) use LaTeX2e or LaTeX 2.09?
I'm a member of the Overleaf support team. Please feel free to send questions like this to Overleaf (email@example.com).
You can check LaTeX version by looking at the compile logs.
You can access these by clicking on the "Logs and output files" button next to the Recompile button, and scrolling down to "View full logs." You'd see something like:
This is pdfTeX, Version 3.14159265-2.6-1.40.20 (TeX Live 2019) (preloaded format=pdflatex 2019.12.13) 14 JUL 2020 16:59 entering extended mode \write18 enabled. %&-line parsing enabled. **sample.tex (./sample.tex LaTeX2e <2019-10-01> patch level 3
Note that you can also check or change the TeX Live version used to compile your project under the "Overleaf menu" of your project.