As so often, I think it just comes down to which is the most usual way of expressing it. "Negatively chronotropic" is correct, in that "chronotropic" means altering the heart rate and "negatively" means lowering it. But in many places, and not just in patient information material but in research articles too, the straightforward and easily understood "heart-rate-lowering" seems to be used.

See, for comparison, the French and English version of the SPC for Gilenya:

"L’expérience est limitée chez les patients recevant Gilenya en association avec des bêta-bloquants, des inhibiteurs calciques bradycardisants (comme le vérapamil ou le diltiazem), ou d’autres médicaments susceptibles de diminuer la fréquence cardiaque [...]
Si un traitement par Gilenya est envisagé, l’avis d’un cardiologue doit être sollicité afin d’évaluer la possibilité d’une alternative par un traitement non bradycardisants avant l’instauration du traitement. Si le traitement bradycardisants ne peut être arrêté, l’avis d’un cardiologue doit être sollicité [...]"
http://ec.europa.eu/health/documents/community-register/2018... (p. 5)

"Experience with Gilenya is limited in patients receiving concurrent therapy with beta blockers, heart-rate-lowering calcium channel blockers (such as verapamil or diltiazem), or other substances which may decrease heart rate [...]
If treatment with Gilenya is considered, advice from a cardiologist should be sought regarding the switch to non heart-rate-lowering medicinal products prior to initiation of treatment. If the heart-rate-lowering medication cannot be stopped, cardiologist’s advice should be sought [...]"
https://www.ema.europa.eu/en/documents/product-information/g... (p. 5)

"Heart-rate-lowering medication" gets a lot of hits, more than any of the alternatives I've tried:
https://www.google.com/search?client=firefox-b-d&ei=Qq7lXNbE...

Including, for example, this from the BMJ:

"In Cox multivariable models mortality increased by 72% [...] after adjustment for age, examination year, alcohol consumption, body mass index [...] and use of HR-lowering medication. [...] In men using and not using HR-lowering medication the risk of death increased 72% [...]"
https://heart.bmj.com/content/94/4/e14

And "heart-rate-lowering treatment" is also reasonable widespread. Here's an article from the European Heart Journal which uses "heart-rate/HR-lowering" quite a lot:

"To date, no human study has been performed to demonstrate the efficacy, the risk-benefit ratio, or even less, the cost-effectiveness of heart-rate lowering treatment in patients without cardiac disorders."
https://academic.oup.com/eurheartj/article/27/20/2387/288729...