Actually 121.5 still uses 100 kHz frequency spacing. It was not affected in the earlier splits from 100 kHz to 50 kHz to 25 kHz, and it's again not affected now.
https://en.wikipedia.org/wiki/Airband
Good try! There are two slightly different things here. The permitted bandwidth of the transmission and the fact that 121.500 is protected by a guard band that can vary by region of the world. The The requirements are laid down in ICAO rules. The bandwith is 25 kHz but the frequency is protected by a guard band that is wider.
The European situation regarding bandwidth is in regulation 1079/2012 where it states 25 kHz channel spacing.
It says in "Amendments to Annex 10, Volume V, Chapter 4" of Oct 2011
4.1.1.1 The block allotment of the frequency band 117.975 – 137 MHz shall be as shown in Table 4-1.
b) 121.5
Emergency frequency
In order to provide a guard band for the protection of the aeronautical emergency frequency, the nearest assignable frequencies on either side of 121.5 MHz are 121.4 MHz and 121.6 MHz, except that by regional agreement it may be decided that the nearest assignable frequencies are 121.3 MHz and 121.7 MHz.
4.1.3.1.6 The emergency channel (121.5 MHz) shall be available only with the characteristics as contained in Annex 10, Volume III, Part II, Chapter 2 for
equipment with 25 kHz channel spacing.
COMMISSION IMPLEMENTING REGULATION (EU) No 1079/2012 of 16 November 2012
Article 2
4. The conversion requirements shall not apply to frequency assignments:
(a) that will remain in 25 kHz channel spacing on the following frequencies:
(i) the emergency frequency (121,5 MHz);