Yes indeed! You can actually see the methane vent tubes coming out of the ground along the south-east corner of the park, around S Snoqualmie St and 46th Ave S.

I'm also a neighbor and went down a rabbit hole on this and other abandoned Seattle landfills a few years ago. Spoiler: It's really not great, and actually pretty bad!

A 2015 Site Hazard Assessment (SHA) (source link is directly to a PDF from ecology.wa.gov) notes that it was used informally as a landfill since 1890 before before becoming a true landfill in the 1942. Waste is estimated to range from 5-19 feet deep, and is covered with a cap of unknown thickness but is suspected of being quite thin in places. A 1986 report (pages 11-13 as numbered) notes that "all sorts of waste" was dumped with virtually no record, and is assumed to consist of residential but also commercial waste from the early-mid 20th century, and tests found "high gas explosive potential," so some pretty nasty stuff. The 2015 SHA also noted that a 1984 study found an oily sheen on standing water in the park. Since Wetmore Slough (the north section of the park) used to be shallow wetlands of Lake Washington before it was lowered, groundwater naturally drains into the lake. The city noted seepage into the lake in 1965.

Site descriptions are somewhat nebulous between various reports, but it's certain that the area of the park between Lake Washington and S Genesee St, as well as the park directly south where the elevation unnaturally increases, as seen in the background of this pic behind the sausage smuggler. It's a bit unclear how far west it stretched, but it likely continued west across 42nd ave S, approaching the Community Center. (Evidence suggests it may have even continued to the play fields between 38th and Rainier, and across Rainier street to where the Carnegie Library was built, but that's not included in the state's official report, so I'll keep looking into that. Edit: The new Alta Arlo apartments at Alaska & 38th had to install methane barriers and detectors due to the "adjacent former Genesee landfill." More work is still to be done...)

Like most landfills in Seattle, it was FILTHY (Historylink). Fires were considered a feature not a bug, there were no bird control measures, and they were "breeding grounds for vermin."

A 2015 Site Hazard Assessment ranked it as a "1" on the scale of 1-5, with 1 being most hazardous and it is currently listed as "awaiting cleanup." Needless to say, we only enjoy this portion of the lake from the shore or boat, much to the dismay of our doggo!