>>43362451As
>>43362484 has said, 5e has made many references to it. Not exactly in a predefined setting, but in more of a way that lets people be reminded that the setting still exists.
The PHB covers the outer planes from Planescape, starting on page 300, though seems to mix in the planar cosmology of other settings with it (Feywild + Elemental Chaos).
MM has quite a few creatures from the Planescape setting as well, Modrons being an obvious one, as well as the Gith (Githzerai and Githyanki), Slaadi, Shuagin (though they are referenced as being native to Baator like they are in Planescape), Tieflings (in their 4e flavor rather than Planescape flavor), and Yugoloths.
And the DMG uses about 35 pages to cover the planes from Planescape, as well as the Feywild and Elemental Chaos. It even goes as far as to directly mention Sigil and the Lady of Pain.
I'd say it's definitely not forgotten, though there isn't really anything new for it.
yet>>43362521I like it.
