D&D 3rd edition (2000-2003)
The five-foot square was not standardized on until D&D 3rd edition, which made it a standard part of the rules in the original core rulebooks published in the year 2000. However, 10ft and hybrid 10ft/5ft squares still appeared in some dungeon maps until the D&D v3.5 revision (2003), which encouraged the designed encounter areas around the use of the 5-foot square combat grid.
The AD&D 2nd edition adventure module Die Vecna Die! (2000), one of the last AD&D books published, still uses 10-foot squares for its dungeon maps, including the map of Adytum on page 20, and the map of Vecna's Palace on page 63. Some maps of large outdoor areas use different scales, such as 25 feet or 30 feet.
The original D&D 3rd edition, nowadays retroactively known as 3.0, made the 5-foot square standard. The 3.0 Dungeon Master's Guide (2000), p.67, "Scale and Squares", states:¹
The standard unit for tactical maps is the 5-foot square. This unit is useful for miniatures and for drawing dungeon maps, which are usually created on graph paper.
In a fight, each Small or Medium-size creatures occupies a single 5-foot square.
However, the sample dungeon map on page 127 still uses the traditional AD&D a scale of 10 feet.
The Player's Handbook v3.0,² inconsistently, does not explicitly use squares, but instead recommends a scale of 1 inch to 5 feet for miniatures at the table (perhaps assuming that while dungeon maps may be gridded, combat will not necessarily be played out with the luxury of a one-inch grid on the gaming table, but instead measured with rulers like a miniature wargame). However, it does specify that Small and Medium creatures have a 5-foot facing, which is the space they occupy in combat.
The D&D 3e adventure module The Forge of Fury (2000) does not mark a scale on its maps, but based on the description of area 9 (30x15 feet for a 6x3 square area), it uses 5-foot squares. This suggests that the mapmakers could assume that five foot square area in D&D 3e was a given. The Speaker in Dreams (2001) also uses 5-foot squares.
Some D&D 3e adventure maps used a kind of hybrid 5-10 foot square, with larger 10 foot squares subdivided into 5-foot squares, or five-foot squares with thicker lines for 10-foot increments. The Sunless Citadel (2000), Heart of Nightfang Spire (2001), Deep Horizon (2001), Lord of the Iron Fortress (2002), and Bastion of Broken Souls used this. Return to the Temple of Elemental Evil (2001) also used this approach, stating "One Large Square Equals 10 Feet".
Dungeon magazine was inconsistent. Dungeon #82, the first 3e issue, uses 5 foot squares in some maps, 10 foot squares in others. Dungeon #95 (Dec 2002) still used some 10 foot square dungeon maps. Lost Temple of Demogorgon, Dungeon #120, used 10 foot squares. Even the last print issue, Dungeon #150 (Sept 2007), still used some 10-foot square maps for very large open areas, such as the Battle of Wat Dagon.
D&D 3.5 (2003) explicitly assumed the use of miniatures and a "1 square = 5ft" scale battle grid (PHB p.4, "Three Dimensions"), with an eye to encouraging the sale and use of the company's miniatures products. The Dungeon Master's Guide v3.5 came with a paper battle grid, and all monster stats thereafter listed movement in 5-foot squares in addition to feet.
This solidified the use of the five foot square in dungeon maps. For example, all indoor maps in Expedition to the Ruins of Greyhawk (2007) use 5 foot squares.
¹ Dungeon Master's Guide, Second printing, September 2001.
² Player's Handbook, Second Printing, November 2002.