Learn Dutch: Is dit oude onderdeel nog vervangbaar? - Is this old part still replaceable?

The suffix -baar (meaning "-able") is attached to the stem of the verb. The infinitive is vervangen (to replace). If you remove the -en, you get the stem vervang. Add -baar, and you get vervangbaar (replaceable).

Onderdeel is a neuter (het) word. Why does oude get an -e ending here, even though het-words sometimes drop it?

Adjectives before neuter nouns only drop the -e when they are indefinite, like after een, geen, or no article at all. Because this noun is preceded by the definite demonstrative pronoun dit (this), the adjective gets the standard -e ending, making it dit oude onderdeel.

Could I use nog steeds instead of just nog in this sentence?

Yes, you could say Is dit oude onderdeel nog steeds vervangbaar? to mean "Is this old part still replaceable?". However, just using nog is perfectly natural and slightly more concise when asking if a possibility still exists.

What is the literal breakdown of the compound word onderdeel?

It is a combination of onder (under or sub) and deel (part). Literally translating to "sub-part", it is the standard Dutch word for a "component" or "part" of a machine, system, or organization.