From this page:
The Vedangas are the last treatises of the Vedic Literature. Paniniya
Shiksha (41-42) narrates two verses on the importance of the Vedangas
which describe Veda as a Purusha having six limbs as six Vedangas:
Chandas are His two feet, Kalpa are His two arms, Jyotisha are His
eyes, Nirukta is His ears, Shiksha is His nose and Vyakarana is His
mouth. The oldest record of their names occurs in the Mundaka
Upanishad (1.1.5) where they are named as:
Shiksha or phonetics or pronunciation
Kalpa or ritual
> Vyakarana or grammar
Nirukta or etymology
Chandas or meter
Jyotisha or astronomy
And the grammar part in brief is explained thus:
The third Vedanga is Vyakarana or grammar, which is necessary for the
understanding of the Veda. It is called the mouth of the Veda Purusha.
The old Vedanga-texts on Vyakarana are entirely lost today. In the
Aranyakas, we find some technical terms of grammar. The only
representative of this Vedanga is the Ashtadhyayi of Panini, which
belongs to a later period. It is indeed the most celebrated text-book
of grammar. It is not associated with any Vedic school. Due to its
great merits, this may be assumed that Panini superseded all his
predecessors, whose works have consequently perished.
Formation of the word is the main subject of grammar. It discusses
root (Prakriti) and suffix (Pratyaya) of a word to study its meaning.
Panini's Vyakarana is in the form of sutras or aphorisms. The fourteen
Sutras are referred to here, as Maheswara Sutras. They were originated
from Nataraja's damuru sound. They are considered the foundation of
grammar. Vararuci has written an elaborate commentary or Vartika. Sage
Patanjali wrote commentary or Bhashya on it.
So without any doubt whatsoever, Sanskrit grammar related questions are on-topic. As it is part of a Hindu scripture and since Hinduism is basically derived from Hindu scriptures, hence it belongs to Hinduism.SE.
Without having knowledge of Sanskrit grammar and how the words are derived from the roots ( which is Nirukta or etymology- another Vedanga) it is not possible at all to understand Hindu Scriptures like Veda etc.
So, by the same logic, all etymological questions are surely on-topic.
Also, whether Sanskrit.SE is there or not has nothing to do with whether Sanskrit grammar related questions will be considered on-topic here or not.
For example, although we have a Mythology.SE, it does not make mythological questions off-topic here.
Apart from the fact, that grammar, etymology are parts of important Hindu scriptures, they are fundamentally important too in understanding the scriptures to start with. And, the subject itself is awesome, fascinating and full of knowledge.
The different interpretation of scriptures come from the grammar-related issues and the different Niruktis of the words being used.
For example, for the word "Purusha", we can have so many different meanings.
He who goes ahead- Purati agre gacchati.
That which fills with its
strength- piprati purayati balam yah.
The dawn in the city; He
which is filled with light (pur+usha).
One filled with wisdom and
eternal happiness (puru-sha).
So, instead coming in way of the knowledge-seekers we should encourage more such questions being asked. Otherwise Hinduism.SE will just be a collection mythological kind of stories which find their places mostly in TV serials and comic books.