(2022):
 
1) New keywords:
       - rate_limit: enforce a reaction rate for each reaction
       - rescale_charges: ensure the total charge of reaction site remains constant
 
2) Support for per-bond variables for custom constraint: 
       - For example, allows bond force to determine when reactions occur

(2021):

1) Advanced reaction constraints:
      - Root-mean-square deviation: geometrical constraint
      - Custom constraint: incorporate values from LAMMPS variables

2) Create new atoms bonded to existing atoms
      - New atoms positioned based on best-fit with reaction site

See the LAMMPS manual for details.