It has been like that for a long time. Many years I believe. There is an option to download clean versions on their website. "Download > Show Additional download options" is the page you are after.
If you check the filename of the windows installer you download from the frontpage the name is FileZilla_Version_Sponsored-setup.exe
The installer available from the link above does not contain the word "Sponsored" and the installer is 2.5 MB smaller.
Additional. Windows Defender tries it best to prevent you from installing the version found on the frontpage due to the adware. It has no issues with the other installer.