Transmission filter looks like the oil filter its usually not replaced but I don't see the harm if you get a new one from Subaru and put it on.
The transmission filter is not supposed to be changed unless the transmission is flushed or repaired. There is no filter change interval specified for it.
And it is not a good idea to change it yourself, because it is hard to tell how much oil the filter held and how much was lost when you unscrewed it.
When you drain the transmission, you can either catch and measure the fluid the came out, or take our word that it is 3.75 quarts, or 6 ounces less than a gallon, or whatever. And you can pour that much new fluid back in and the dipstick will look right the first time you check it.
But if you also remove the filter, you are going to be into buying a 4th quart of expensive ATF, and not know exactly how much of it to put in. So you will funnel some fluid in, and then check the dipstick, which must be done at hot idle, and then funnel in some more. And if you overshoot and put in too much, how will you get it out? Or would you just leave it in?
Better leave that transmission filter alone. It was for a good reason that Subaru deleted it in 2008. Too many ignorant Jiffy Lubes or unprepared owners removed it, and the aftermath was cars running with underfilled or overfilled transmissions.