Apps Script has turned down the legacy integrated development environment (IDE) in favor of the redesigned IDE that launched in December 2020.
Google have announced the updated Apps Script editor will replace the legacy experience by Q4 2022. Whilst the new IDE was a huge step forward there are some core features from the old Script Editor, which will hopefully make it into the new IDE before Q4. The ones I hear most often are access previous script versions and search/replace across project script files. For the later in the meantime I recommend looking at the Black Apps Script Chrome extension. Whilst this extension primarily enables a true dark mode it also lets you search across script files.