QueryStore plan forcing limitations
When looking at the plan-forcing limitations one of the bullet points is:


  Distributed query or full-text operations


However, this limitation does not mean that a delete should fail the plan forcing.

Consider these 4 queries, two SELECT statements and two DELETE statements on a table with a fulltext index on the val column:

SELECT * 
FROM  dbo.fulltextindexed;

SELECT val 
FROM  dbo.fulltextindexed
WHERE CONTAINS(val, N'WOW');

DELETE top(1)  
FROM dbo.fulltextindexed;

DELETE top(1)  
FROM dbo.fulltextindexed
WHERE id &lt;500 and id &gt; -1;


The only query that is failing is the 2nd one, which is using the CONTAINS statement:



With the failure reason being DQ_NO_FORCING_SUPPORTED.

The plans for the two delete statements, which also show the same clustered index merge operators get forced:



This source gives more information on DQ_NO_FORCING_SUPPORTED:


  Cannot execute query because plan conflicts with use of distributed
  query or full-text operations


To answer the question


  Does this join to the FT index mean that plan-forcing is not
  supported?


My answer would be that this is not the reason as you should see the DQ_NO_FORCING_SUPPORTED instead of the NO_PLAN error. 
The reason could be due to another limitation rendering the plan invalid, index changes, added foreign keys, ....



Extra tests

When adding a table and creating a foreign key with on delete cascade that references the table used in the queries above.We would expect the delete to fail since we cannot use the forced plan any longer.

CREATE TABLE dbo.fulltexreference(id int identity(1,1) primary key not null, t1id int, val varchar(max));
ALTER TABLE dbo.fulltexreference add constraint FK_test foreign key(t1id) REFERENCES dbo.fulltextindexed(id) ON DELETE CASCADE;


As expected, we get the NO_PLAN on the two delete statements


Re-forcing the plans gives us salvation:



And removes the NO_PLAN issue



The same is true for a table with a fulltext index that has a foreign key (with on delete cascade) that references the main table where the delete query is running on.

Regarding the high / low estimates

If the delete is not executed that much, adding OPTION(RECOMPILE) could help with the high row estimates as this can give you a better estimation due to the optimizer 'seeing' the variable at runtime. 

Posting a different question with the query plan might bring up a different workaround / solution.