As Robert mentioned in the comments, this depends on certain parameters sent to Google Play when requesting the download link. Based on my research, the key parameter is Vending.version, which specifies the version of Google Play that is making the download request. The last version that can request the app without split APKs is 80913000, which represents a specific Google Play version. After this version, it’s no longer possible to download a solid APK.

Additionally, some apps have implemented restrictions within themselves, so even setting this parameter to 80913000 won't allow the app to be downloaded. You can see this in the link below, where, starting from a certain app version, even on ApkCombo, downloading a unified APK file is no longer possible:

https://apkcombo.com/space-takeover-over-city/risk.city.dominations.strategy.io.games/download/apk