Exactly When Did The US Stop Issuing Soakable Self-Adhesives Stamps?

In the late 1990s and early 2000s, the US issued self-adhesive stamps that were soakable, that is...you could soak them and the adhesive would remain attached to the envelope while detaching from the back of the stamp. Does anyone know exactly when or why the USPS stopped doing this? I'm sure that it was some sort of cost issue. Also, does anyone know the Scott number of the last stamp such as this? I'm curious so I'll know which of the stamps of this era that I can soak and which ones will require the Pure Citrus treatment.

Somewhere around the 37c rate, as I recall.

Initially U.S. self-adhesive stamps included a water-soluble layer … this allowed the stamps to be soaked from an envelope. The USPS found it was cheaper to exclude this layer … hence the stamps we have now that cannot be successfully soaked. Scott began using a symbol in their U.S. catalogues to indicate whether a self-adhesive stamp was considered soakable. As I recall it is a red S enclosed in a black circle. I'm not certain whether the symbol indicates that the stamp is soakable or not soakable. You would need to consult the catalogue legend for that info.
Correction: Black S enclosed in a red circle.
Added: Took a while, but I located the info. Scott includes a footnote following Scott 1549 … the symbol is a warning to NOT soak the stamp. Either the stamp will not separate from the backing paper (no water-soluble layer) or the stamp will be subject to damage (e.g., ink may flake off).

JLLebbert, Thank you for this information, it was exactly what I was looking for. I was unaware that the Scott catalog contained this information. The earliest stamps that I saw with such a symbol during the time frame I'm interested in were the Constellation issues of 2005, beginning with Scott number 3945.

I found seven earlier ones.
(1) Scott 3863, the 2004 Olympics issue; a footnote indicates that the ink can crack & flake off during soaking for this stamp.
(2) Scott 3293, the 1999 Sonoran Desert pane of 10 stamps (part of the Nature of American series).
(3) Scott 3013, the 1995 32-cent Children Sledding stamp; footnote indicates that some adhesive may remain after soaking.
(4) Scott 2919, the 1995 32-cent Flag over Field (ATM pane of 18); footnote like (3).
(5) Scott 2887, the 1994 Old Glory G-rate stamp from ATM pane of 18; footnote like (3).
(6) Scott 2522, the 1991 plastic Flag F-rate stamp; footnote like (3).
(7) Scott 2475, the 1990 plastic 25-cent Flag; footnote like (3).

JLlebbert, Thanks, you're the man!