Depends on the type of diabetes, the treatment and the level of knowledge.
Knowledgeable and experienced type 1 (juvenile) diabetics with an acceptable BMI do not have any restrictions at all. This was different 25 years ago.
For type 2 (adult-onset) diabetes it depends. Knowledgeable and experienced ones with an acceptable BMI relying on insulin injections do not have any restrictions at all. This was different 25 years ago. Those who rely on diet and exercise should avoid high-glycemic food including too much sugar in coffee or tea. Of course avoiding high-glycemic food is good for all inactive people.
Otherwise coffee and tea has little to do with diabetes.
Knowledgeable diabetics don’t suffer. The term “diabetes sufferers” is biased and certainly not appropriate. It creates stigmas. A better way of asking this is:
Is drinking tea or coffee ok for diabetics?