Not exactly as plain text but if I assume a CSV format will work for you then you can leverage The Stack Exchange Data Explorer.
There is a small problem. On Stack Overflow there are over 50,000 tags at this moment. SEDE limit the number of rows in a single resultset to 50,000 so you either have to do two passes or use a little trickery to get more then 50,000 rows in one CSV. I have chosen to do the trickery part here.
This SEDE query uses a common table expression to get all tags and then uses a union which allows to split the resultset in two columnsets. The first column set for the rows 1 - 50,000, the second columnset for rows 50,001 and higher.
declare @cutoff int = 50000 -- this is the max
;with alltags as
select row_number() over (order by id) [rownum]
select max(id) id
, max(tagname) tagname
, max(count) count
, max(excerptpostid) excerptpostid
, max(wikipostid) wikipostid
, max(id2) id
, max(tagname2) tagname
, max(count2) count
, max(excerptpostid2) excerptpostid
, max(wikipostid2) wikipostid
, null id2
, null tagname2
, null count2
, null excerptpostid2
, null wikipostid2
from alltags firstSet
where rownum < @cutoff
(rownum - @cutoff + 1)
from alltags SecondSet
where rownum between (@cutoff) and (@cutoff * 2)
-- add extra unions here and double the number of fields
group by rownum
order by [rownum]
Once you've run this query you can download the results by clicking the Download CSV link found near the top right of the result grid:
An alternative is using the relatively new SQL instruction
FOR JSON AUTO. This will give you a one column result set where each row is part of a complete JSON. Once you have collected these rows you need to stitch them all together and then parse that JSON text. Benefit of these approach is that you can output much more rows in one go. For example the Tags table now only requires 2,410 rows.
Here is that query in its full glory:
for JSON AUTO
So with some post-processing you have all tags for one site.
Keep in mind SEDE is updated once a week on Sunday.
Use the awesome SEDE Tutorial written by the magnificent Monica Cellio.
Say "Hi" in SEDE chat.