Using the command /data merge block ~ ~1 ~ {Text1:'{"text":"testing"}'} with a sign at eye level in past versions would add the text "testing" to the sign but does nothing in this version. Likewise, using the command /give @p minecraft:oak_sign{BlockEntityTag:{Text1:'{"text":"testing"}'}} would give me a sign with the text "testing" but in this version gives me a blank sign
Linked issues
Comments 2
Duplicate of MC-263051.
In 1.20, signs were updated so that they are now able to have text on both faces. This has changed how the NBT data is written, thus why it is no longer working for you.
The following command is a display of the proper command usage now:
(Generated using a sign generator)
If you'd like to learn more about this change, you can here.