Interestingly, treatment of mild pharyngitis before the advent of antibiotics wasn't too different from what it is today - it was (and still is) mostly symptomatic. First of all, viral pharyngitis, which is quite common (Hidreth et al., 2015), is not affected by antibiotics (no viral infection is), and they are not indicated in such situations. Second, mild and self-limiting bacterial infections, including pharyngitis, most often do not warrant prescription of antibiotics, as this leads to the growing problem of antibiotic resistance, which, according to the World Health Organization is "one of the biggest threats to global health." Many guidelines can be quoted, urging clinicians to prescribe antibiotics sparingly, but pharyngitis is so common (literally everyone has had it, multiple times) that one can simply turn to one's own experience - simply put, when you have a mild case of soar throat, your doctor doesn't give you antibiotics (if you even bother going to the doctor about it), most often you are advised to take only symptomatic medications (many of which are OTC). These most often include compounds with analgesic, antiseptic, and/or anti-inflammatory properties. Interestingly, such compounds were widely available long before the advent of antibiotics and even the pharmaceutical industry itself.
Menthol, for example, is a selective agonist of κ-opioid (Galeotti et al., 2002) and a modulator of GABBAa receptors (Watt et al., 2008), thereby eliciting analgesic and anaesthetic effects, respectively. Moreover, by disrupting bacterial cell membranes, menthol (and similar compounds, such as thymol, for example), elicit antibacterial activity, as well (Trombetta et al., 2005). Thus, wild mint, peppermint, and thyme are much more often indicated in cases of soar throat than antibiotics! Their use in the form of infusions and decoctions likely predates most, if not all, modern countries. Moreover, the analgesic and anti-inflammatory properties of willow bark, which are due to the salicylic acid contained therein, were known to Hippocrates (Norn et al., 2009), quite possibly dating back much earlier. Furthermore, it is known that caffeine potentiates the analgesic effects of many nonsteroidal anti-inflammatory drugs, including salicilic acid (Granados-Soto and Castañeda-Hernández, 1999). Unsurprisingly, this combination is often found in many modern drugs (many OTC), being marketed all over the world. Whether this effect was known before the advent of the pharma industry, however, is hard to say. Willow bark has been used as an analgesic and anti-inflammatory medication since ancient times, but coffee was brought to the West in the XVI century.
This is only a handful of traditional medications for pharyngitis, and I'm sure many more can be found, as well as many other treatments for many other conditions. The point is that treatments of mild pharyngitis were available before the advent of antibiotics, as a matter of fact, many of those treatments are still first-line therapy in most cases, even to this day.