Simplest latching circuit with press-ON hold-OFF button that can cut power when Raspberry Pi halts
If you don't mind small packages and having to order through a major distributor...

If you can work with a small (TSOT-23-8 with a 0.65mm pitch) SMT package (or use a suitable breakout board such as a Capital 33608 or Proto-Advantage PA0089), and don't mind spending a few bucks on this, then I would use a LTC2954-2, a logic-level PFET, and a handful of passives (resistors and capacitors) to get this job done; it's going to be the lowest-component-count and easiest-to-get-right solution to this problem while providing a reliable GPIO interface to the Pi.  A minimal application circuit is shown below, where M1 can be any logic level (i.e. with its Rds(on) specified at -5V Vgs) P-channel MOSFET capable of handling >3A. (The FDC640P is in a SMD package about the same size as the LTC2954-2; if you want a through-hole part instead, an Alpha &amp; Omega AOI21357 is a suitable substitute.)







simulate this circuit &ndash; Schematic created using CircuitLab