The Ail and An Arc isn't filer!
I'm dead serious.
Whenever an anime is an adaptation of a manga, anime fans are so used to labelling anything not featured in the manga as 'filler' they do so without thinking.
Filler is called 'filler' because it is superfluous. It is filling in time and is therefore skippable. The reason we have 'filler free' lists for various anime is specifically so people can watch an anime and follow the main plotline in a speedier manner.
The problem with labelling the Ail and An arc as filler is that you absolutely HAVE to watch it to follow the main narrative!
Alright, not literally ALL 13 episodes. But you for sure need to watch some of the episodes. There is simply NO WAY you can jump from the end of the Dark Kingdom arc in season 1 to the debut of Chibi-Usa in season 2/Sailor Moon R without feeling lost.
Season 1 of the anime concludes with Usagi, Mamoru and the Inner Senshi not remembering their adventures as super heroes nor their relationships with one another. If someone jumped from that to the first episode of the Black Moon Clan arc they'd either be confused about or need to headcanon the following:
How and when did Usagi and the gang remember their lives in season 1
How and when did Usagi and Mamoru get together?
How and when did Usagi get this new 'Moon Crystal Power' transformation and this new 'Moon Princess Halation' attack?
Whilst it is true the Ail and An arc is nowhere to be found in the manga, there is no end of elements in the 1992 anime which are also not to be found in the manga that the fandom en masse doesn't label as filler.
Naru and Nephrite's relationship
The hunt for the Rainbow Crystals
Kunzite and Zoisite's relationship
Minako's backstory with Alan and Katarina
The Inner Senshi dying at the clutches of the DD Girls
Usagi's showdown with Super Beryl
All those are iconic plot relevant to the over arching storyline of the first season alone and none of it is in the manga, not even in a 'squint and you will see it' kind of way.
I will also add that some episodes of Ail and An arc are skippable but do great character work in their own right. The Usagi and Mamoru babysitting episode for example subtly foreshadows what the rest of the season will revolve around, namely Usagi and Mamoru's relationship with their own child, Chibi-Usa.
