Is the Transaction Selection Strategy a known attack on Kaspa's GHOSTDAG?

No — the Transaction Selection Strategy is a known property of all inclusive protocols, not a novel attack on GHOSTDAG. In blockchain research, an "inclusive protocol" is one that accepts parallel or competing blocks rather than discarding them. This characteristic — that miners can influence which transactions get included — was already identified and discussed in academic literature as far back as 2014, in a paper on Inclusive Block Chain Protocols co-authored by researchers who are also involved with Kaspa. For a beginner, the takeaway is that this is not a newly discovered vulnerability; it is a documented, studied trade-off that the protocol's designers were already aware of.