Many players assumed the 20th anniversary event would have a Partyhat for 2 reasons:
1)
The 15th Anniversary party hat is a party hat which could be created by players who collected 100 party hat fragments during the 15 Year Anniversary Celebrations. The fragments could be received by popping the balloons dropped at all lodestones.
2)
Mod Warden's Wintumber Letter 2020
Just make sure you keep hold of those party hats - you never know when they may be worth something! We'll see each other again very soon.
I suppose after all the trolling Jagex has done with Phats, it's really no surprise people assumed one may actually show up for a huge event like 20th Anniversary. Regardless if
a Black Phat had actually showed up, it would have been untradable since the wiki says:
Jagex's stance on partyhats
Jagex have stated that they do not wish to recreate the effect that partyhats have had on RuneScape's economy. This is the reason that tradeable holiday drops were replaced with events with non-tradeable rewards. After releasing the fish mask, Jagex changed its stance on tradeable discontinued items, with the mask being the first tradeable discontinued item released since the Santa Hat.
Mod Mark said that if a large number of discontinued rares fell victim to a botnuke, Jagex would look at ways to re-introduce them, probably via the Grand Exchange.
Happy Anniversary Runescape! Met my wife on this game, which ironically, is why I don't play it much. Thank you for all the memories and Happy New Year everyone!