It should be in force as of 2020-07-01: From the Belarusian Telegraph Agency, reported on 2020-06-19:
INSK (sic), 19 June (BelTA) – Belarus and Russia have concluded an intergovernmental agreement on mutual recognition of visas and other issues related to the entry of foreign nationals and stateless persons to the territory of the states participating in the Union State Treaty, BelTA has learned.
However, another source (in Russian) claims that although it has been signed, it will still take several more months before it is actually in force.
Old situation (before 2020-07-01)
On 10 July 2019, the Belarusian Telegraph Agency wrote:
The agreement on mutual recognition of visas is technically ready, awaits a political decision, State Secretary of the Belarus-Russia Union State Grigory Rapota said in an interview with the Belarus 1 TV channel on 9 July, BelTA has learned.
On 18 November 2019, they quoted Belarusian foreign minister Vladimir Makei:
“The agreement is almost ready. I am convinced that the decision on it can be adopted in the near future,” Vladimir Makei said.
So, as of 18 November 2019, it is not yet in force, and no date has been publicly announced on which it will be, only the vague “near future”.