I didn't notice this when in came out (August 2019) but NIST has updated SP 330 to the 9th edition. It is essentially the identical to the 9th Edition SI Brochure from BIPM with Americanized spelling (meter, liter, deka-, cesium, metric ton, L as the symbol for liter).

The prior edition (unlike the SI Brochure, 8th edition) accepted obsolete radiation units( curie, roentgen, rad, rem) to be used in the US with the SI, as part of Section 4, although the usage was strongly discouraged. That section is entirely dropped.

The companion usage guide, NIST SP 811, has not yet been updated to the new SI definitions. If you want a (pdf) copy, follow the NIST link in the right hand Resources column. I'm not looking to start a metre/meter war, but the government agency in charge of interpreting the metric system for the US does officially tell Americans to spell this way, so most of us do. The good news is that either spelling is still 1/299 792 458 of a light second, unlike the gallon.