I have a 1999 Legacy Wagon, 30th Anniversary Edition. Engine Type EJ223AXAWL, Transmission Type TZ1A2ZAEAA, currently at 100,345 miles. Transmission fluid was original. It started having significant delays shifting into D from P or R, similar to all the other stories in this post, in the last 3-months. I had not researched this subject on the internet prior to going to the Subaru dealer.
The week before going to the dealer saw many occurrences of the delay. The day before going, I had the battery out and the car in a slightly nose-down attitude for over an hour (cleaning out the mess left by the last battery...I didn't want to pay the shop to do this easy task). After that, I experienced no delays and the shop technician saw a perfectly-behaved transmission as well. The dealer performed an AT & Torque Converter flush & devarnish and filled with new fluid.
I have driven 93 miles since having the battery out (65 miles since picking it up from the dealer) with about a dozen delay-free shifts (plus however many the technician performed). I am mentally prepared to replace some fluid with Trans-X, but am waiting for the delays to return first.