You stil can buy reservations to have a seat on these trains for sure. 

There are usually above or at the seats small displays that show between which station this seat is reserved.  Worstcase if a train is full you  have to stand. just plane spare times for delays or other problems along the route i usually plan my routes with atleast one backup train in cases of delay or track closure. 

In CH and AT you will always be able to board. You can in fact only REServe IC-trains.

In DE and ONLY for ICE trains (or the odd other exception) there are some bans on overcrowding-these also depend on actual route (tunnels or not). You can REServe for just 4€/ride as ´seat only´ on bahn.com-noone will check this if you sit on right seat/train/day, and its all electronic

BUT BUT-this summer there will be a 9€/month ticket for ALL DE-local trains/bus/metro etc, and it is expected that on busy (summer)days some trains=notably to beach/lake/mountain can get extremely crowded and police may hve to come to clear it. Wait&see. This ticket is of corus emuch cheaper as IR-pass, but only for DE as a 1-time off measure (against high fuel and inflation)

Oh-also please note that DB has the last 2 -3 yrs a very awful record of not being on time-quite different from what most people think of the Deutsche. So to reach that airport=Flughafen do take ample time and do not plan closely-and preferably not over long distance with ALternative of bus or tram if the need arises. Also sudden huge crowds after covid has gone makes for much longer time needed to clear security etc. in airports. 

Nearly all airports in DE now have some type of S-bahn=local to reach, the main ones also a long-distance stop (notably FRA, also CGN=KólnBonn, but NOT München)

Again not true about AUSTRIA. There are regularly on busy days trains stopped and people without reservations are kindly requested to leave the train… if not enough people leave, the police is called and they will throw people without a reservation from the train. 
Are you @mcadv the master of fake news??

@OP: you can avoid this by making a reservation. They won’t throw you out if you have a reservation. But still train will be delayed for sure in such a case.