I have written extensively about the use of decongestants in the past ( https://breastfeeding-and-medication.co.uk/fact-sheet/coughs-colds-flu-and-covid-when-breastfeeding and https://www.breastfeedingnetwork.org.uk/factsheet/decongestants/).
Pseudoephedrine has been shown to reduce milk supply in some women. The effect of phenylephrine on supply has not been demonstrated but use of oral decongestants is generally not advised for breastfeeding women.
I have always recommended the use of nasal sprays as decongestants together with the use of steam inhalation as being more effective than oral medication and with no potential impact on supply.
In September 2023 the FDA reported that oral phenylephrine is not effective at relieving nasal stuffiness. It is important to note that neither FDA nor the Non-prescription Drug Advisory Committee raised concerns about safety issues with use of oral phenylephrine at the recommended dose. (https://www.fda.gov/drugs/drug-safety-and-availability/fda-clarifies-results-recent-advisory-committee-meeting-oral-phenylephrine).
According to one USA website the evidence for efficacy was first questioned in 2007. The manufacturers have cited a survey that many people find its use beneficial as shown by sales volume of cough and cold products.
The vote that formally declared phenylephrine ineffective was in line with a review of pharmacology and clinical data presented by the FDA, which found the oral bioavailability of the drug is less than 1%, compared with 38%, a number often cited in the literature and based on outdated technology. (Medscape https://www.medscape.com/viewarticle/996369).
There appears to be no current UK recommendations although I am sure there is current discussion. Most oral cough and cold remedies currently contain phenylephrine as pseudoephedrine sales have been restricted. Between 2007 and 2008, the government introduced restrictions on their use because of concern that medicines containing these active substances could be used in the illicit manufacture of the Class A controlled drug methylamphetamine https://www.gov.uk/drug-safety-update/pseudoephedrine-and-ephedrine-update-on-managing-risk-of-misuse#:~:text=Sales%20restrictions,-Since%20April%202008&text=It%20is%20illegal%20to%20sell,mg%20ephedrine%20without%20a%20prescription
This information has been compiled from a variety of sources and does not imply recommendation other than that nasal decongestants and steam inhalation are effective in reducing symptoms of nasal congestion during breastfeeding without potential impact on supply. Most of us have our own preferred remedies which we find effective and that remains a personal choice