It can be either. When it is used as a slippery slope or taken to extremes it becomes a fallacy. Taking one extreme case that is implausible is an example of how it become a fallacy rather than a technique trying to prove something by contradiction.
Reductio ad absurdum is a method, a tool, a technique to reveal the absurdity in someone's logic. It's a way to reveal fallacies, but it, itself is not a 'fallacy'. I think you are calling the result of the application of the technique a 'reductio ad absurdum fallacy'. I suppose I can live with that as long as you understand that your 'fallacy' was the result of applying the technique to find it, but the distinction must be stated that there is no such thing as a stand alone 'reductio ad absurdum fallacy', which is to say it's not a thing, if using the entire phrase as a noun, but using 'reductio ad absurdum fallacy,' where the method is being used as an
adjective, (using it as that which categorizes the fallacy) that could work. The simplest way to describe the 'method' is to take whatever idea or proposition you are offering and make sure you follow it all the way through to it's farthest logical outcome, at which point you'll discover it's validity, or absurdity.
When this technique is applied to 'Democrats rigged the election', the absurdity of that claim becomes apparent.
The key to this approach is to explore the logical consequences of such a claim: If Democrats really rigged the election, it would imply an extraordinarily massive and well-coordinated conspiracy across multiple states, involving election officials, courts, media outlets, and various levels of government who would have had to be organized months in advance and preplanning. Where are the 'whistleblowers? THere are none. See,. Not only would that involve millions of people keeping quiet, a practical unreality,. but it would also have to withstand countless recounts, audits, and legal challenges without any substantive proof emerging. Following this line of reasoning to its extreme conclusion reveals how far-fetched it is, exposing the claim’s inherent absurdity.
This method works well in debates because it invites the claimant to either defend increasingly untenable positions or concede the fallacy of their argument. So, the use of reductio ad absurdum here is a valid way to dismantle the "Democrats rigged the election" narrative by showcasing how implausible it becomes when logically extended.
