The new Installer doesn't install anything in FSX or P3D, the "Install" function is only available for MSFS 2020, if you tried it, it would have told you so.
This is not a problem, since all FSX/P3D installer are always freely available on our site and they already do what you asking, the Install functionality was more important for MSFS 2020, because there we don't have a Trial version, so downloads were hosted by Digital River, and they have a fairly limited re-download policy of 30 days, unless you pay extra to buy a Download Protection from them, but even that one maxes out at 2 years, so we needed to fix this.
All the other functions are identical between MSFS and FSX or P3D, so you can still use it to Activate, Deactivate, Get your keys, read the manuals and, of course, keep the products Updated.
It's likely we'll add the Install functionality to FSX and P3D as well, but we don't have any release date now, because it's not urgent as it was for MSFS.