Oracle 18c

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create table registered (
crn number,
term varchar2(6));
create table course(
crn number,
term varchar2(6),
term_descrip varchar2(25));
begin
insert into registered values (123,'202101');
insert into registered values (456,'202001');
insert into registered values (789,'202101');
insert into registered values (123,'202001');
insert into registered values (456,'201905');
insert into registered values (789,'202101');
insert into registered values (246,'202101');

insert into course values (123,'202001','Anatomy');
insert into course values (123,'202101','Physics');
insert into course values (456,'200001','English');
insert into course values (456,'201901','Algebra');
insert into course values (789,'199901','Gym');
end;
/
1 rows affected

select r.crn, r.term, c.term_descrip
from registered r 
  outer apply(
    select *
    from course c
    where c.crn = r.crn
      and c.term <= r.term
    order by c.term desc
    fetch first 1 rows only
  ) c


CRN
TERM
TERM_DESCRIP


123
202101
Physics

456
202001
Algebra

789
202101
Gym

123
202001
Anatomy

456
201905
Algebra

789
202101
Gym

246
202101
null

CRN	TERM	TERM_DESCRIP
123	202101	Physics
456	202001	Algebra
789	202101	Gym
123	202001	Anatomy
456	201905	Algebra
789	202101	Gym
246	202101	null