Scholars say that the voluntary prayers that the Prophet salla Allahu alaihi wa sallam used to pray them regularly are called recommended Sunnah.
The prayers that the Prophet salla Allahu alaihi wa sallam told us to do but didn’t maintain it himself, these are called not recommended Sunnah.
The Prophet salla Allahu alaihi wa sallam used to not offer some deeds so that he wouldn’t burden his Ummah if he does them regularly.
Therefore, the word not recommended doesn’t mean that it is disliked. It means here that he didn’t pray it regularly.