A red triangle could mean a lot of things, so maybe best to ask people using it what their intention is...
Examples of possible meanings could include:
'Emergency!" - red triangles have often been used to draw attention to an emergency or hazard - eg a humanitarian crisis.
Political prisoner or silenced - historically, red triangles like that were used by Nazis to identify political prisoners and communists (I think) in concentration camps - some people have adopted it as a way of expressing that they feel disillusioned with their government or unable to speak freely.
Peace - some cultures use it as a symbol for peace.
Solidarity with women - it can be used to represent or celebrate femininity or standing with women.
Support for the people of Palestine - the Palestinian flag includes a red triangle.
But yes, one potential meaning may be expressing support for the events of October 7th. Propaganda including released video footage of the attacks included the use of downwards pointing red triangles to mark the positions of targets/victims. It's a symbol that has been used in online propaganda - cashing in on its other associations, wider use in emergency appeals, and potential to de-humanise (video games may use similar symbols for target acquisition - using it may be an effort to encourage particularly young men to see violence as more of a game and less something to stand against).
If I see someone carrying this symbol, I would ask them what they mean by it. Unless it's clearly in the 'Emergency Appeal' context, I'd explain that they may want to find alternative symbols to express their meaning. I wouldn't assume everyone with one in their social media profile was pro-terrorism, though, especially when it's a symbol with such a range of potential meanings.
The easiest way to find out what someone means by something is to ask them.